The present invention relates to a control device of a diesel engine with a turbocharger.
Conventionally, diesel engines provided with diesel particulate filters (hereinafter, also referred to as “DPF”) in their exhaust passages have been well known. The DPF captures particulate matters (PM) within exhaust gas, and when an accumulation amount of the PM increases, it needs to be regenerated. Normally, on the upstream side of such DPF, a catalyst having an oxidizing function (e.g., oxidation catalyst) is provided and the catalyst is used in the DPF regeneration. For example, JP2004-0316441A discloses a diesel engine that performs a main injection in which fuel for generating a torque is injected into a cylinder, and then a post injection to introduce uncombusted fuel to an exhaust passage. When the uncombusted fuel reaches the catalyst, it is oxidized and increases an exhaust gas temperature. As a result, the PM accumulated in the DPF is combusted to be removed by a high temperature exhaust gas. Thus, the DPF is regenerated.
Moreover, this kind of diesel engine may be provided with a turbocharger to collect an energy of exhaust gas and increase a turbocharging pressure. For example, JP2009-0191737A discloses a diesel engine that includes two large and small turbochargers. The turbine of the small turbocharger is arranged upstream of the turbine of the large turbocharger in the exhaust flow. Moreover, in an exhaust passage of this diesel engine, an upstream bypass passage for bypassing the upstream turbine and a downstream bypass passage for bypassing the downstream turbine are placed, a regulator valve is provided within the upstream bypass passage, and a waste gate valve is provided within the downstream bypass passage.
Meanwhile, in the engine with the turbocharger disclosed in JP2009-0191737A, when the engine shifts to a deceleration state (low-speed-and-load operation state), it is preferred that the regulator valve is controlled to have a smaller opening and the small turbocharger with comparatively less rotary inertia is operated. Thereby, an acceleration response can be improved when reaccelerating the engine.
However, in this case, the exhaust gas discharged from the cylinder of the engine is supplied to an oxidation catalyst after passing the turbine of the small turbocharger, and thus, the heat of the exhaust gas is taken by the turbine and the exhaust gas temperature to be supplied to the oxidation catalyst decreases.
Especially, when the engine is in a deceleration state, a fuel injection to a cylinder on compression stroke (main injection) is prohibited (because a fuel cut is performed), and therefore, the temperature of the exhaust gas to be supplied to the oxidation catalyst significantly decreases, causing a difficulty in maintaining the oxidation catalyst in an activated state. Therefore, even if the post injection is performed to regenerate the DPF, the injected combusted fuel will not be oxidized, and thus, an exhaust gas temperature increasing effect using heat from the oxidation reaction is lost and it takes time to regenerate the DPF, causing degradation in fuel consumption and problems that a larger amount of fuel attaches on an in-cylinder wall face because the post injection and engine oil is diluted.